UC San Diego

Large Eddy Simulation (LES) approach is utilized to investigate submesoscale (0.1 - 10 km) dynamics at isolated upper-ocean fronts, with a focus on vertical transport and its interaction with the turbulent finescale (0.1 - 100 m). The study considers an unforced front and also a more realistic scenario with surface cooling.

In unforced simulations, the evolution of baroclinic mixed layer instability spawns coherent vortex filaments and eddies, which are similar to the coherent features observed during the Lagrangian Submesoscale Experiment (LASER) in the Gulf of Mexico. Turbulence is generated locally in the vortex filaments. Because of localized turbulence, a 2D low-pass filter in the horizontal with cutoff wavelength at O(100 m) is used to decompose the flow into submesoscale (quasi-2D) and finescale (3D) components to study their energy interaction explicitly. The decomposition also helps understand the organization of persistent upwelling and downwelling regions at the front. Material transport is further examined by releasing Lagrangian tracer particles. Analyzing particle trajectories, we identify an eddy/lobe/filament framework that describes the coherent transport of particles at the front and elucidates the restratification process. The sub-inertial time scale observed for the vertical motion of particle clouds suggests that submesoscale dynamics control vertical transport and restratification. Dispersion characteristics of the turbulent submesoscale currents using single-, pair- and multi-particle statistics are investigated.

Simulations with surface cooling show that coherent structures are preserved in the face of mixing by boundary-layer convective turbulence. We find a strong two-way coupling between submesoscale currents and convection. Convective plumes are able to develop through vortex filaments, but are suppressed in other parts of the front. The submesoscales are also affected. The organization of vertical velocity changes, the front develops greater imbalance, and the release of potential energy by the submesoscale buoyancy production increases. Cooling assists the downscale energy cascade by significantly enhancing the conversion from submesoscale to finescale velocity.